The use of UV radiation to inactivate microorganisms in fluid is known There are several types of UV systems including those that are pressurized, semi-pressurized or not pressurized. Such systems generally rely on lamps positioned in rows submerged in the fluid to be treated, such as drinking water. Regardless of the type of UV system, it is important to deliver as close to an equal amount of UV light (dose or fluence) to all fluid flowing though the system to inactivate any microorganisms. This dose or fluence is equal to the product of the intensity and time. As microorganisms pass though the UV system they are subjected to a range of UV intensities and times resulting in a distribution of doses. Ideally the UV light and flow are distributed evenly across the UV reactor resulting in all microorganisms receiving the same dose. The object in designing UV reactors is to achieve a narrow dose distribution where all fluid elements are exposed as close to this ideal dose as possible.
To achieve such ideal dose distribution, some systems have offset successive rows of UV lamps so that the fluid passes through the spaces between the lamps in the first row and contacts the lamps in the second row. However, a concern in such systems is absorption of UV light by adjacent lamps because light cannot pass upstream and downstream unobstructed. This method can also impose higher headloss and require more lamps, albeit of lower power.
Another method to ensure that the flowing fluid is subjected to a range of UV intensities is to locate a flat baffle parallel to the lamps. In some methods baffles are positioned between each set of UV lamps. The baffles direct the fluid to pass relatively close to the lamp or lamps. This method results in a higher pressure drop through the reactor and leaves zones behind the baffles with low or virtually no flow resulting in high doses in these areas, especially when the UV transmittance of the water is high. It therefore becomes difficult to design a reactor that achieves a narrow dose distribution over the full range of water UV transmittances that the reactor is called on to treat.
Other systems, position lamps in a predominantly circular array to improve UV dose distribution (as shown for example in FIG. 3a). However, even in these systems, and UV systems using similar patterns, there are areas where water does not receive a minimum exposure to the UV fluence rate field. Such systems provide a fluid distribution where some of the fluid receives a low dose of UV and other fluid receives a high dose yielding a wide dose distribution and therefore the potential for microorganisms receiving lower doses to pass though without being inactivated.
Systems configured with a circular array of UV light sources may also tend to lack efficient lamp turndown capabilities. Typically, circular array systems require the system controls to turn off pairs of lamps, rather than lamps individually, to maintain symmetry with as close to an even close distribution as possible. Further, in the circular array, turning off two lamps results in irradiance gaps in the flow stream in areas where the lamps are off, as compared to radiance emitted in areas where the remaining lamps may be close to each other. The uneven dose distribution yields poor efficiency and, in some cases, inadequate treatment levels or untreated water. Some prior systems require large units to administer required doses.
Thus there is a need to eliminate the shortcomings of the prior art including the undesirable effects of a non-uniform treatment dose distribution. It is further desirable to provide a UV reactor that has a compact design, good operating efficiency, efficient flexible turndown when turning lamps off and is low in cost.